FIGS. 1 and 2, which show a plan layout and cross-sectional view, respectively, illustrate a conventional way to lay out a connection device 10 on a semiconductor substrate 12 having a dielectric layer 14 overlying the substrate 12 through which a via 16 is formed to facilitate electrical connection with conductive line 18 on substrate 12. Via 16 is filled with a conductive member 20, which may be understood to be a conductive plug, e.g. of tungsten, to which metal line 22, e.g. of aluminum, is connected.
FIG. 2 is a cross-section of the connection device 10 layout of FIG. 1 taken at line 2--2 in FIG. 1 illustrating the connection device 10 from the side. In the finished semiconductor device, there are additional dielectric and passivation layers provided over the connection device 10 and metal line 22 to seal and protect them.
Due to the confinement of metal lines by dielectric layers and the difference in thermal expansion coefficients, the metal lines in semiconductor devices are under high thermal stress. This stress will tend to drive the vacancies or holes out of the metal interior to coalesce at the end of the metal line to minimize the strain energy.
Further, because of repeated flow of electrons in a metal line predominantly in one direction, there is repeated electromigration of the metal atoms in the direction of the predominant current flow. S. M. Sze, ed., VLSI Technology, McGraw-Hill, New York, 1983, pp. 369-371, indicates that
"[a] prime consideration in device reliability is the electromigration resistance of the metallization. Electromigration is observed as a material transport of the conductive material. It occurs by the transfer of momentum from the electrons, moving under the influence of the electric field applied along the conductor, to the positive metal ions. Hence, after a conductor failure, a void or break in the conductor is observed and a nearby hillock or other evidence of material accumulation in the direction of the anode [Figure references omitted.] is found."
On page 370, Sze shows several SEM micrographs of breaks in metal lines. Techniques to increase electromigration resistance of aluminum film conductors include alloying with copper, incorporation of discrete layers such as titanium, encapsulating the conductor in a dielectric or incorporating oxygen during film deposition as reported by Sze. The medium-time-to-failure (MTF) of the conductor also seems to be related to the grain size in the metal film; distribution of grain size; the degree to which the conductor exhibits fiber texture, i.e. in the &lt;111&gt;direction; method of film deposition and line width, according to Sze.
FIG. 3 is a cross-section of the connection device 10 of FIG. 2 after it has been subjected to numerous thermal cycles and predominantly unidirectional current where it is seen that metal line 22 has pulled away from conductive member 20 to create a break or open in connection device 10. There are prior art structures that use more than one member 20 to connect metal line 22 with conductive line 18, but such a structure is for the purpose of handling the current density. Thus, if the metal line 22 retracts from one of the conductive members 20 and is still in connection with remaining conductive members 20, the remaining conductive members 20 may be insufficiently large to handle the current density. The stress migration and electromigration phenomena thus accelerate to create an open. Thus, there is a high potential to have voids formed at the end of a line, especially if the line carries appreciable current or is relatively long. These problems are more serious for devices stressed at high temperatures and/or high current densities.
It would be helpful if other straightforward methods were discovered which reduces electromigration and increases MTF for metal conductors.